This application claims the priority of Korean Patent Application No. 2002-49295 filed on Aug. 20, 2002 in the Korean Intellectual Property Office, the contents of which are incorporated herein in their entirety by reference.
1. Field of the Invention
The present invention relates to a display device, and more particularly, to a method and circuit for driving a panel of a thin film transistor liquid crystal display device using low power.
2. Description of the Related Art
A circuit for driving a thin film transistor (hereinafter referred to as a TFT) liquid crystal display (hereinafter referred to as an LCD) device is generally classified into a gate driver circuit and a source driver circuit.
FIG. 1 is a view of a general TFT-LCD device. Referring to FIG. 1, the general TFT-LCD device includes a liquid crystal panel 110, a gate driver circuit 120, and a source driver circuit 130.
The liquid crystal panel 110 includes a liquid crystal, storage capacitors CST, and switches ST. The liquid crystal may be modeled as a liquid crystal capacitor CL. Thus, the liquid crystal panel 110 may be modeled as a structure in which liquid crystal cells 111 having a liquid crystal capacitor CL, a storage capacitor CST, and a switch ST are arranged as many as the number L of channels in the row and as many as the number of lines in the column.
A node of the liquid crystal capacitor CL is connected to a corresponding switch ST. The switch ST is an MOS transistor having a gate to which a voltage output from the gate driver circuit 120 is applied. The gate driver circuit 120 turns on/off gates of the switches ST.
The source driver circuit 130 inputs a gradation voltage (or a gray scale voltage) corresponding to display data to the liquid crystal. if switches in a specific line are turned on by the voltage output from the gate driver circuit 120, the gradation voltage output from the source driver circuit 130 is applied to the liquid crystal capacitor CL connected to the turned on switches. The storage capacitors CST are capacitors used to reduce current leaking from the liquid crystal.
Of the gate driver circuit 120 and the source driver circuit 130, the source driver circuit 130 accounts for a large portion of the whole power consumption. In particular, in the source driver circuit 130, driver amplifiers 131 through 13L, which form ends of channels for actually driving the liquid crystal, consume a large portion of power. Thus, reducing power consumption in the source driver circuit 130, particularly, in the driver amplifiers 131 through 13L, is the most efficient method of reducing the power consumption of the whole driver circuit.
FIG. 2 is a view of the driver amplifier 131 shown in FIG. 1.
The power consumed in the driver amplifier 131 is classified as static power and driving power. The static power is consumed by a constant current IS for stably driving the driver amplifier 131. The driving power is consumed by a driving current ID for driving a liquid crystal capacitor and a storage capacitor.
The power consumption of the driving amplifier 131 is obtained by Equation 1.P_TOT=PS+PD=IS×VDD+CL_EFF×VOS×F  (1)wherein, P_TOT is the whole power consumption of the driver amplifier 131, PS is the static power of the driver amplifier 131, PD is the driving of the driver amplifier 131, IS is the constant current of the driver amplifier 131, CL_EFF is the equivalent capacitance for the liquid crystal capacitor and the storage capacitor, VDD is a power voltage, VOS is a voltage difference in an operation section of an output voltage VOUT of the driver amplifier 131, and F is an operation frequency of a display device.
In Equation 1, since the driving power PD of the driver amplifier 131 depends on a load CL_EFF of the liquid crystal panel and the operation frequency F of the display device, the driving power PD is limited to being reduced. Thus, the power consumption P_TOT of the driver amplifier 131 can be reduced by reducing the static power PS by the constant current IS of the driver amplifier 131.
The configuration of the driver amplifier 131 shown in FIG. 1 will be described in more detail with reference to FIG. 3. Referring to FIG. 3, the driver amplifier 131 generally includes an amplifying stage 131_1 and a driving stage 131_2.
The constant current IS in the driver amplifier 131 having the configuration shown in FIG. 3 is classified into a bias current IB flowing in the amplifying stage 131_1 having an input differential pair and a driving stage constant current IQ flowing in the driving stage 131_2 for driving a large load. The bias current IB with a compensation capacitor CC determines a slew rate of the driver amplifier 131 as in Equation 2. The driving stage constant current IQ determines a transconductance gm of driving transistors PM1 and NM1 of the driving stage 131_2 and affects a phase margin representing the stability of the driver amplifier 131.
                    SR        =                  IB          CC                                    (        2        )            wherein, SR is the slew rate of the driver amplifier 131, IB is the bias current IB, and CC is the capacitance of the compensation capacitor CC.
In a case of a driver amplifier used in an existing TFT-LCD driver circuit, the bias current IB determining the slew rate is designed so as to satisfy a driver output setup time characteristic required in the worst case, i.e., if the output voltage VOUT of the driver amplifier 131 swings at its maximum.
FIG. 4 is a view illustrating output characteristics of a driver amplifier according to the prior art.
As described above, the driver amplifier according to the prior art is designed so as to satisfy a driver output setup time tD required when an output voltage VOUT of the driver amplifier swings at its maximum. That is, in FIG. 4, the slope of the output voltage VOUT has to satisfy G1. Thus, even when the output voltage VOUT of the driver amplifier does not greatly vary, the slope of the output voltage VOUT G2 is equal to G1. In this case, the driver output setup time tD is reduced more than necessary. Therefore, a bias current of a more than admissible value flows in the driver amplifier, which results in an increase in the whole power consumption of an LCD driver circuit.
Accordingly, in order to reduce the power consumption, it is preferable that in a case where the output voltage VOUT of the driver amplifier does not vary greatly, the slope G2 of the output voltage VOUT is gentle as shown in FIG. 4 compared with a case where the output voltage VOUT swings at its maximum. That is, it is preferable that the slew rate of the driver amplifier be low in terms of power consumption.
Since a driver amplifier for driving an LCD device according to the prior art uses a fixed slew rate regardless of variations in an output voltage, power is unnecessarily consumed.